Proteins having repetitive sequences (protein repeats) of theoretically infinite length can be synthesized by carrying out rotary protein translation using cyclic RNA not having a stop codon as template (a method of synthesizing a protein having an endless peptide repeat structure (also referred to as rolling circle translation). In the case of ordinary translation systems, the rate-limiting step of protein synthesis is initiation of peptide elongation after ribosomes have recognized and bound RNA.
For example, protein repeats are synthesized in Non-Patent Document 1 by a translation reaction of cyclic RNA using an Escherichia coli cell-free translation system. More specifically, rotary protein synthesis is carried out by using cyclic RNA, in which the protein open reading frame (ORF) is arranged downstream from a sequence consisting of a Shine-Dalgarno (SD) sequence, start codon (AUG) and downstream box (DB), as template.
In eukaryotic cell translation systems, the translation reaction is initiated by ribosomes recognizing and binding a complex formed at the start of the translation reaction by various translation factors binding to a cap structure on the 5′-terminal of mRNA and to a poly A chain on the 3′-terminal. Consequently, in the case of using cyclic RNA not having a cap structure or poly A chain as template, ribosome binding sites (structures able to be recognized and bound by ribosomes) are thought to be required that are capable of functioning in place of these structures. Therefore, in Non-Patent Document 2 and Patent Document 1, for example, cyclic RNA is used as template in which an internal ribosome entry site (IRES) derived from a virus (EMCV) is arranged upstream from the start codon in a rotary protein translation system that uses a cell-free translation system derived from rabbit reticulocytes.